Hydra attenuata, a fresh water coelenterate, has the ability to regenerate its entire form from a fragment of body tissue. The animal is basically a cylinder composed of a double-layered epithelium. At the apical end is a cone-shaped hypostome or mouth with a ring of tentacles at its base, and at the basal end is a disc of sticky cells. During the regeneration of an almost uniform sheet of tissue into a complete animal, hydra exhibits a spectrum of properties characteristic of embryological fields. It thus provides a simple model system for studying the pattern formation during development. Regnerated pieces of tissue will be analyzed to determine how various parts of the pattern are generated, and what their interrelationships are. Specific questions posed: 1. Does the coordination found during simultaneous regeneration of the two organizers, the hypostome and basal disc, arise from interactions between separate patterning systems, or does a single patterning system control the development of both? 2. Does proportion regulation of the basal disc occur with the same precision and range as found for the head, and if so, how does their mutual size determination relate to the shape of the body column? 3. Is the emergence of periodic structures, the tentacles, a regulative pattern where the number of repeating elements is maintained, or a spacing pattern where distance between elements is controlled, or both depending on the size range of the regenerates?